Recent developments in the use of polymers of substituted alkynes have focused interest on the preparation of such monomers. For example, poly-1-(trimethylsilyl)-1-propyne is very attractive in preparing membranes for gas separations. The monomer is prepared by metallating propyne with butyllithium followed by reaction with chlorotrimethylsilane. The fact that propyne is very expensive has adversely affected the economics of these substituted acetylenic polymers. One approach to reduce the cost of such polymers and other derivatives of substituted alkynes has been to use an impure source of alkynes as a feedstock. This approach is illustrated in U.S. Pat. No. 3,752,848, Smith (1973), which describes making tetrolic acid (2-butynoic acid) by introducing carbon dioxide into a slurry of propynyl alkali metal, such as propynyl sodium, and then hydrolyzing the carbonated slurry. The patent states that the unrecovered product slurry produced by the method of U.S. Pat. No. 3,410,918, Beumel, et al. (1968) can be used as the source of the propynyl alkali metal compound after disappearance of the alkali metal particles and cooling to below 80.degree. C. The '918 patent describes the preparation of propynyl sodium and propynyl lithium by reacting propyne with sodium or sodium/lithium dispersions. A mixture of propyne and allene in a weight ratio of about 1:1 to 4:1 is used as the feedstock and the allene remains relatively inert so as not to interfere with the propyne metalation. Some propyne is said to be hydrogenated to propane. The propyne/allene feedstock is cheaper than pure propyne.
The feedstock such as that referred to by the '918 patent is actually available commercially as a cutting fuel and is referred to as MAPP gas, having a composition of about 23 to 36% propyne, 18 to 28% propadiene and 1 to 8 wt. % propylene and butenes.
Although it is known that propadiene can be isomerized to propyne, this approach has not been used to increase the value of such mixed feedstocks. U.S. Pat. No. 3,671,605, Smith (1972) discloses isomerizing allenes into acetylenic isomers at temperatures of -10.degree. to 100.degree. C., using a catalyst of sodium or potassium reacted with alumina. Lithium is said to be inoperable. Alumina is reacted with molten alkali metal with agitation and under an inert gas blanket. The allene, such as propadiene, may be pure or mixed with its acetylenic isomer, for example, propyne. In either case the product is said to be an equilibrium mixture in which the acetylenic isomer predominates.
Dykh, et al., Izv. Akad. Nauk SSSR. Ser. Khim. (1978), 11, 2473, disclose that allene can be isomerized to methyl acetylene on metal oxide and zeolite catalysts. Various aluminas were used at temperatures of 20.degree. to 350.degree. C. Isomerization of methyl acetylene to allene also occurs and the kinetics of the forward and reverse reactions are discussed.
Khulbe, C. P.; Mann, R. S., Can. J. Chem. (1978) 56, 2791, disclose equilibrium constants for the isomerization of allene to methyl acetylene and discuss this reaction catalyzed with silica-supported cobalt and iron.
The isomerization of allenes is also disclosed in U.S. Pat. No. 4,036,904 Strope (1977) which describes purifying a 1,3-butadiene stream prior to catalytic cyclodimerization when the stream contains allenes which would poison the dimerization catalyst. Allene and 1,2-butadiene are converted to acetylenic compounds over a magnesium oxide catalyst at 85.degree. to 355.degree. C. Allene is converted to methylacetylene and 1,2-butadiene is converted to 2-butyne or 1,3-butadiene.
Brown, C. A.; Yamashita, A., J. Am. Chem. Soc., (1975) , 97, 891 disclose that potassium 3-aminopropylamide rapidly catalyzes the isomerization of alkynes having interior triple bonds to 1-alkynes. Macaulay, S. R. Can. J. Chem., (1980) , 58, 2567 discloses sodium aminopropylamide with somewhat higher temperatures is more effective than the reagent of Brown, et al. The materials studied were decyn-1-ol and its isomers. Abrams, S. R., Can. J. Chem., (1982) , 60, 1238 states that isomerizations reported by Brown and Yamashita can be carried out with catalysts which are sodium salts of 1,3-diaminopropane or 1,2-diamino ethane. Use of the reagents is disclosed for isomerizing acetylenic acids.
Abrams, S. R., Can. J. Chem., (1984) , 62, 1333 describes an improvement in the catalysis of triple bond migration in isomerizations to form terminal alkynes and alkynols over the earlier work with catalysts which were sodium salts of 1,2-diaminoethane or 1,3-diaminopropane. The improved catalysts are lithium salts of these compounds with the addition of sodium or potassium alkoxides, such as potassium tert-butoxide.
In summary, the conversion of alkadienes by isomerization to terminal acetylenes is well known and a variety of isomerization catalysts are available for this procedure. Such isomerization of allenes is also suggested in Wotitz, et al. J. Org. Chem. (1973) 38, 489 and by Abrams. et al., J. Org. Chem. (1987) 52, 1835.